In the field of magnetic heads for disk drives, large advances in signal sensitivity have been made in recent years with the employment of magnetoresistive sensors. Such sensors utilize elements having a resistance to electrical conduction that changes in response to an applied magnetic field, in order to read signals such as magnetic patterns on a disk. Generally such elements comprise at least one thin layer of ferromagnetic material that is magnetized in a reference direction in the absence of an applied magnetic field. As such a sensor is exposed to an applied magnetic field, the magnetization direction of that layer changes from the reference direction, and the resistance to electrical current also changes, which is measured as a signal. Various mechanisms are known for establishing the reference direction and signal bias, including the use of a permanent magnet, canted current, soft adjacent layer or an antiferromagnetic pinning layer.
Magnetoresistive sensors can include anisotropic magnetoresistive elements, giant magnetoresistive elements or spin valve elements. Spin valve sensors conventionally employ a pinned magnetic layer separated from a free magnetic layer by a conductive spacer layer. When the magnetization of the free layer is parallel to that of the pinned layer, it is believed that parallel electron spins of the magnetic layers allow conduction to occur more easily than when the magnetizations are not parallel. The magnetization of the pinned layer is conventionally held fixed by an antiferromagnetic layer that adjoins the pinned layer.
Unless the pinning force is quite strong, however, the applied magnetic field can alter the direction of magnetization of the pinned layer as well as rotating the free layer magnetization, denigrating signal resolution. Moreover, the coupling between the pinned and pinning layers becomes weaker at higher temperatures, exacerbating the problem of having a pinned layer with a magnetization that may not be fixed. Resistive heating of the sensor during operation can lead to such a breakdown. Further, a breakdown of coupling at elevated temperatures can allow a shift in the direction of magnetization of the pinned layer upon cooling, leading to further problems in reading and interpreting signals. Such a shift can also mischaracterize servo tracking information, causing heads including the sensors to have offtrack errors.
Another form of pinning that has been proposed is to use a balanced pair of oppositely magnetized layers with an extremely thin (a few angstroms thick) layer of a noble metal (ruthenium) sandwiched between the oppositely magnetized layers. This balancing can reduce the magnetic moment felt by the pair of magnetic layers compared with the moment that would be felt by only one of the magnetic layers. An antiferromagnetic layer adjoins one of the magnetized layers for pinning the sandwich. The necessity of forming extra layers, one of which must be as thin as a few atomic layers of ruthenium, however, makes large scale manufacture of this proposal extremely difficult.